1. Field of the Invention
The present invention relates to a testing system which simulates road induced vehicle vibration and loads for testing vehicle front wheel drive components.
2. Description of the Prior Art
In the prior art the use of road simulators has been recognized as being desirable, and various road simulators that will test axles and spindles have been advanced. The problems associated with road simulators has been outlined in an article entitled "Making A Road Simulator Simulate", by Ronald A. Cripe (SAE No. 720095, 1972). In particular, the importance of loading the test specimen using a simulated rolling tire configuration, and the problems associated therewith are discussed.
The "wheel hop" phenomena is also discussed in that article. Wheel hop can be explained briefly as the loading which occurs when the tire leaves the road. The vertical load on the pavement is thus reduced, and therefore the horizontal load drops off substantially. Simulating this loading under the various spring rates that are encountered on the road, particularly during steering motions, has proved to be a substantial problem.
Further, the problems associated with cross coupling of forces in vertical and horizontal inputs when testing front wheel axles has long been recognized. Devices for compensating the cross coupling effects mechanically and electrically have been advanced. For example, U.S. Pat. No. 3,713,330, issued in Jan. 1973 to Thomas Lentz for an Axle Test Device, greatly simplifies the mechanical arrangement required for force cross coupling compensation, and uses bell crank loading.
U.S. Pat. No. 3,718,033, issued Feb. 27, 1973 to Petersen, for a Testing Apparatus Utilizing Position-Force Cross Coupling Control teaches dynamic cross coupling compensation in vertical and horizontal loading actuators by adding a compensating signal to the servovalve control for the horizontal loading actuator, which signal is dependent upon the position of the vertical actuator. These devices both work well for testing spindles, but some of the loading problems, such as simulated wheel hop, are not solved by such devices.
A test system that includes fixtures to apply loads in additional axes is shown in U.S. Pat. No. 4,263,809, issued in Apr. 1981 to Petersen et al., and this too uses cross coupling compensating load linkages and bell crank loading devices.
In addition to wheel hop, problems in testing wheel support components arise because of a torque steer effect that is described and discussed in U.S. Pat. No. 4,479,382, issued to Greenhorn et al., on Oct. 30, 1984 and entitled System For Testing A Tire To Avoid A Torque Steer Effect.
The effect of torque steer effect is minimized in the present device by having a single base that provides for steering and all horizontal loading is tied back to this base in the present device.